FIG. 1 illustrates a prior art oscilloscope input circuit. The input signal is applied through a traditional BNC connector 10 to a unity-gain buffer amplifier 12 which typically has a field effect transistor (FET) input stage. The input signal is terminated by a 1 MΩ resistor to ground which provides a defined input impedance. A variable capacitor CADJ can be adjusted to trim the input capacitance. The circuit of FIG. 1 can be characterized as a voltage-mode circuit because the buffer amplifier directly measures the voltage across the termination resistor.
FET amplifiers are used because of their high sensitivity and high input impedance, which is generally beneficial when attempting to measure high-frequency, low-level signals. This sensitivity, however, makes them vulnerable to transient and overload conditions. Since oscilloscopes are used for research, development, testing, etc., they are necessarily connected to signal sources having unknown parameters. Such signal sources may apply transients measured in kilovolts (kV), as well as other types of extended overloads, to the input circuit. Therefore, a transient/overload protection circuit 14 must be included before the input amplifier. Protection circuits, however, are complicated and expensive. In fact, the cost and space consumed by protection circuitry tends to overwhelm that of the actual input measurement circuitry. Moreover, the protection circuitry tends to interfere with the normal operation of the measurement circuitry.